It is actually easy to spot (see the table here): in Phred + 33, capital letters are at the top end of the quality range (Phred score plus 33 gives an ASCII code for a capital letter when Phred is 32 to 42), while a capital letter would be an abysmal score in Phred +64. And in your case, you have "#" which is only valid for Phred +33 (ASCII code 35).